[Local] collectBitParts - reduce maximum recursion depth.

As noticed on D90170, the recursion depth for matching a maximum of a i128 bitwidth was too high.

@lebedev.ri mentioned that we can probably do better by limiting the number of collected Values instead of just depth, but I'll look at that later.

llvm/lib/Transforms/Utils/Local.cpp

@@ -111,8 +111,8 @@ static cl::opt<unsigned> PHICSENumPHISmallSize(
         "perform a (faster!) exhaustive search instead of set-driven one."));
 
 // Max recursion depth for collectBitParts used when detecting bswap and
-// bitreverse idioms
-static const unsigned BitPartRecursionMaxDepth = 64;
+// bitreverse idioms.
+static const unsigned BitPartRecursionMaxDepth = 48;
 
 //===----------------------------------------------------------------------===//
 //  Local constant propagation.